A lot of it is habit. A lot more of it comes from not really taking notice of what one is doing. This seems to increase as we age. You'll probably have noticed that as people grow older their repetoire of personal stories begin to narrow down. By the time they're eighty or so the score of stories they once told, reduced to a dozen, then half-a-dozen and, finally, to maybe just one or two. It's as if a robotic machine is gradually having its recorded memory. It's wiped off - and maybe it is.

Answers: develop good habits. Be aware. Trust that nothing is ever forgotten, recall being the weakness. And just accept that as we grow older the increasing 'plaque' in our brains brings about a gradually slowing in that recall.